The present invention concerns a method for the production of electrodes for lead storage batteries and the resulting electrodes. More specifically, the invention concerns electrodes and a method for forming electrodes where the active material in the form of lead powder is made to form a connected or continuous structure before or while it is applied to a supporting grid.
The starting material for the production of active material to form lead electrodes is called "lead powder", which term is somewhat inaccurate as the lead powder consists of lead powder of varying degrees of oxidation, and generally between 70 and up to 100%. By degrees of oxidation is meant the percentage of the total weight of the mixture which consists of lead oxides. As used herein, "lead powder" is intended to mean this highly oxidized lead powder mixture as known in the art. The lead powder is applied to a grid which usually consists of a diamond pattern or a number of connected ribs made of lead or lead alloys. There occur great variations, however, in the grid constructions, both with regard to their formation and the choice of material. A grid may, for example, consist of a number of suitably formed lead wires, or it may contain parts made of plastic or some other metal besides lead. Lead grids are usually produced by means of casting, but they can also be formed from expanded metal or stamped sheeting.
With certain electrode types such as the so-called tube electrodes and similar electrode types where the active material is surrounded by an outer sheath, the active material is applied on the grid in the form of dry powder. With such electrodes as well as with other electrode types the active material may also be applied as a so-called lubricating mass. This is accomplished, as is known in the art, by mixing lead powder with water and some sulfuric acid, and perhaps smaller amounts of other additions. In this way, a lubricant paste is obtained. In this paste, the lead powder forms a coherent structure. Its porosity depends on the addition used, and in order to obtain a high porosity, which is desirable in certain cases, special pore-makers may be added.
A special problem in connection with lead storage battery electrodes is caused by the volume variations that occur with both the positive and the negative electrodes. This is due to the fact that the active material in the case of the positive electrode in charged condition is composed of lead dioxide, and in the case of the negative electrode, of pure lead, while in the discharged condition the material in both electrodes is present in the form of lead sulphate. These different compounds have different volumes per unit of mass which causes a tendency to increase the volume of active material in electrodes with a subsequent separation of the active material, which forms a precipitate and sinks down to the bottom of the storage battery cell. This separation of active material in turn leads to a reduced capacity of the electrodes and shortens their life span. In order to avoid this problem, the electrodes, while being built into their respective cells, have been provided with supports of different kinds, e.g., with separators provided with glass fiber mats or sheets that are in contact with the electrodes. Such devices, however, can be relatively expensive, and even though these have led to the desired result, at least to a certain extent, additional improvements in this respect remain desirable.